12 June 2009

Change in monsoon patterns likely

by Kate Melville

An abrupt change in climate in the past was associated with a shift of seasonal monsoons to the south, causing more rain to fall over the oceans than in the Earth's tropical regions, leading to a dramatic drop in global vegetation growth. The researchers behind the new findings, from Oregon State University (OSU), the Scripps Institution of Oceanography and the Desert Research Institute in Nevada, have published their study's findings in the journal Science.

They say that if similar changes were to happen to the Earth's climate today - as many scientists believe is possible - it would lead to drier tropics, more wildfires and declines in agricultural production in some of the world's most heavily populated regions.

The team's findings are based on oxygen isotope measurements from ice cores, and are supported by previously published data from ancient stalagmites found in caves. The data confirming these effects were unusually compelling, the researchers said. "Changes of this type have been theorized in climate models, but we've never before had detailed and precise data showing such a widespread impact of abrupt climate change," said OSU's Ed Brook. "We didn't really expect to find such large, fast environmental changes recorded by the whole atmosphere. The data are pretty hard to ignore."

The researchers used oxygen measurements, as recorded in air bubbles in ice cores from Antarctica and Greenland, to gauge the changes taking place in vegetation during the past 100,000 years. Increases or decreases in vegetation growth can be determined by measuring the ratio of two different oxygen isotopes in air. They were also able to verify and confirm these measurements with data from studies of ancient stalagmites on the floors of caves in China, which can reveal rainfall levels over hundreds of thousands of years.

"Both the ice core data and the stalagmites in the caves gave us the same signal, of very dry conditions over broad areas at the same time," Brook explained. "We believe the mechanism causing this was a shift in monsoon patterns, more rain falling over the ocean instead of the land. That resulted in much lower vegetation growth in the regions affected by these monsoons, in what is now India, Southeast Asia and parts of North Africa."

Worryingly, previous research suggests that the climate can shift quite rapidly - in periods as short as decades or less. This study provides a barometer of how those climate changes can affect the Earth's capacity to grow vegetation. "Oxygen levels and its isotopic composition in the atmosphere are pretty stable, it takes a major terrestrial change to affect it very much," Brook said. "These changes were huge. The drop in vegetation growth must have been dramatic."

"These findings highlight the sensitivity of low-latitude rainfall patterns to abrupt climate change in the high-latitude north," the researchers note in their report, "with possible relevance for future rainfall and agriculture in heavily-populated monsoon regions."